A method for the prevention of disturbances and/or the effects of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen

Abstract

 A method for the prevention of disturbances and/or effects of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen or a molecular oxygen-containing gas at elevated temperature by means of introduction of a basic substance into the reaction mixture when an uncontrolled temperature increase occurs during the oxidation.

Description

[0001] The invention relates to a method for the prevention of disturbances and/or the effects of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen.

[0002] It is known that hydroperoxides of hydrocarbons, such as for example isobutane, cyclohexane, cumene and ethyl benzene, can be prepared by passing oxygen or an oxygen-containing gas through the relevant hydrocarbon at elevated temperature. To shorten the induction period and improve the selectivity the reaction is usually carried out in the presence of a basic substance.

[0003] U.S. patent specification 2,632,772 mentions as suitable basic substances, inter alia, the hydroxides and carbonates of alkali metals, the oxides and hydroxides of alkaline earth metals, the normal phosphates of said metals, and ammonia. In U.K. patent specification 713,138 secondary or tertiary amines, in particular pyridine, are used and in Netherlands patent specification 6810123 the use of alkali metal pyrophosphates is recommended, preferably in such quantities that the alkali metal content of the oxidation mixture is 0.1-100 ppmw.

[0004] It is known, for example, from Netherlands patent application 7511955 that in the oxidation of hydrocarbons with a molecular oxygen-containing gas a greatly accelerated decomposition of the hydroperoxide (known as a runaway) may suddenly occur. A possible cause thereof may be that small quantities of substances which catalyze the decomposition of the hydroperoxide find their way into the reaction system. Since the decomposition of the hydroperoxide is attended by a high degree of heat development, it is of great importance that the rise in temperature caused by the accelerated decomposition is controlled as soon as possible, since otherwise an explosion might take place.

[0005] In Netherlands patent application 7511955 this problem is discussed with reference to an example regarding the oxidation of cumene. According to said patent application, in the processes in which cumene is oxidized not a single effective method was known to control a dangerous rise in temperature in time and use was made only of membrane cooling surfaces having limited effectiveness.. Netherlands patent application 7511955 therefore proposes a process for the prevention of disturbances and/or the effects of disturbances in the oxidation of hydrocarbons in the liquid phase under pressure with oxygen-containing gases, in which process water is introduced into the reaction vessel in an intensive way, preferably in a quantity which is necessary to cool the liquid hydrocarbon to below the boiling point at atmospheric pressure of the hydroazeotrope of the hydrocarbon to be oxidized or to a temperature at which no uncontrolled decomposition of the hydroperoxide takes place. The water is preferably introduced in a period of 0.5-5 minutes. It is, of course, obvious that said process, in which large quantities of water are sprayed into the reactor, involves great practical drawbacks.

[0006] It has now been found that a sudden undesirable rise in temperature in the oxidation of hydrocarbons with molecular oxygen can be controlled in a considerably less drastic manner. The invention relates to a method for the prevention of disturbances and/or the effects of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen or a molecular oxygen-containing gas at elevated temperature, characterized in that if an uncontrolled rise in temperature occurs in performing the oxidation, a basic substance is introduced into the reaction mixture. If the reaction is already being carried out in the presence of a basic substance, an extra quantity of a basic substance should be added. It is surprising that by adding a usually small (extra) quantity of a basic substance to the reaction mixture at the moment when the uncontrolled rise in temperature starts, the latter can be checked.

[0007] The quantity of the basic substance to be added is generally small and is mostly between 0.05 and 20 gram-equivalents, in particular between 0.1 and 5 gram-equivalents, of the basic substance per 1000 kg of the reaction mixture present in the reactor. The use of smaller or larger quantities of the basic substance, however, is not excluded.

[0008] According to the invention both inorganic and organic basic substances can be used. Examples of suitable basic substances are hydroxides, carbonates, bicarbonates, phosphates or pyrophosphates of alkali metals or alkaline earth metals, ammonia, salts of alkali metals and organic carboxylic acids, such as acetic acid, and amines, for example dimethylamine, trimethylamine, triethylamine, dibutylamine, triethanolamine, piperidine, pyridine and tetraethylenepentamine. The basic substance can be added in gaseous, liquid or finely divided solid state or in the form of a, for example aqueous, solution.

[0009] The basic substance can be introduced into the reaction mixture by addition to the hydrocarbon feed, to the oxygen or the oxygen-containing gas or to the reaction mixture itself. In any case care should be taken that the basic substance is completely mixed with the reaction mixture as quickly as possible, preferably within 5 minutes. This can be effected by stirring vigorously, using stirring systems present in the reactor. In order to ensure that the mixing is completed rapidly, the basic substance is preferably added to the oxygen or the oxygen-containing gas. In this case it will be preferred to use a basic substance which is vaporous at room temperature or at least at the temperature at which the oxygen or the oxygen-containing gas is passed into the reaction mixture. The possibility of atomizing the basic substance in liquid, dissolved or finely divided solid state into the oxygen-containing gas stream, however, is not excluded. Suitable bases which are vaporous at room temperature are, for example, dimethylamine, trimethylamine and ammonia. The latter compound has particular preference. Very good results can be obtained by adding for example 1-300, preferably 2-80 ppmw of NH₃,based on the weight of the reaction mixture present in the reactor, to the oxygen or the oxygen-containing gas. The use of smaller or larger quantities, however, is not excluded.

[0010] Immediately after the basic substance has been introduced into the reaction mixture, the rise in temperature discontinues. To accelerate a reduction in temperature in the event of an uncontrolled rise in temperature, it will be preferred also to start up cooling systems present in the reactor. Consequently, the reaction mixture can optionally be cooled to a temperature at which practically no further reactions take place. In addition, the supply of oxygen or of the oxygen-containing gas will preferably also be reduced or closed. The reaction mixture which is present in the reactor and is optionally cooled can then be passed to the next step of the process and be processed further in the usual manner. Subsequently, the reactor can be filled with fresh hydrocarbon and the oxidation can be continued. However, in a continuous process, after the addition of the basic substance, it is in principle possible to continue the passing through of the oxygen or the oxygen-containing gas. Of course, measures must then be taken to remove the cause of the runaway, for example by switching over to another feed. In such a case, especially if the reaction mixture is not cooled, it may be necessary to repeat the addition of base once or several times when taking the relevant measures, if an uncontrolled rise in temperature again takes place.

[0011] The method according to the invention is particularly suitable for use in the preparation of hydroperoxides of tertiary alkanes, cycloalkanes and aralkanes. Said hydrocarbons preferably contain 4-20 carbon atoms. The aralkanes may contain one or more aromatic rings optionally substituted with one or more alkyl groups. Examples of suitable hydrocarbons are isobutane, isopentane, isohexane, 2,3-dimethylbutane, cumene, ethylbenzene, ethyltoluene, ethylnaphthalene, cyclopentane, cyclohexane, methylcyclohexane and cyclododecane. The temperature at which the oxidation is carried out depends on the hydrocarbon to be oxidized and is mostly between 80 and 160 C. The oxidation of, for example, cumene is generally carried out at a temperature between 80 and 140°C, and in the oxidation of ethylbenzene a temperature between 135 and 160°C is mostly used. The oxidation is mostly carried out at a pressure between 1 and 70 bar abs.

[0012] The moment when an uncontrolled rise in temperature takes place can easily be determined by any operator. The oxidation of, for example, ethylbenzene can very suitably be carried out at a constant temperature of 150°C. If the temperature suddenly rises to, for example, 152°C without apparent cause, this may signify that undesirable decomposition of the hydroperoxide is taking place and that the method according to the invention must be used.

[0013] Consequently, the invention also relates to a process for the preparation of a hydrocarbon hydroperoxide in which oxygen or a molecular oxygen-containing gas is passed through a hydrocarbon at elevated temperature, characterized in that in order to prevent disturbances and/or the effects of disturbances a basic substance is introduced into the reaction mixture if in the course of the reaction an uncontrolled rise in temperature takes place.

EXAMPLE I

[0014] A reactor in which a distributor for the introduction of a gas and a stirrer had been installed, was charged with 1 litre of ethylbenzene to which 1.5 ppm of sulphur in the form of 2,5-dimethylthiophene had been added. Under the conditions in which the oxidation is carried out, the latter compound can catalyze the decomposition of the hydroperoxide formed. At a temperature of 150°C and a pressure of 3 bar abs., 80 1/h of a mixture consisting of air and nitrogen was passed through the ethylbenzene. The ratio between the quantities of air and nitrogen was adjusted in such a way that the oxygen concentration in the off-gas was 4% by volume. After 90 minutes the temperature suddenly started to rise and was 152°C after 15 minutes (total reaction time 105 minutes). At this moment 10 ppm of NH₃ (based on the weight of the reaction mixture) wsre added to the mixture of air and nitrogen. In spite of the fact that no measures were taken to reduce the temperature of the reaction mixture and that the passing through of the oxygen-containing gas was continued, the temperature immediately started to fall and reached the original value of 150°C after 10 minutes (reaction time 115 minutes). After a reaction time of 125 and 150 minutes quantities of 10 ppm of NH₃ were once more added. The results are summarized in the following Table A which also states the phenol content of the reaction mixture, a measure for the decomposition of the hydroperoxide.

[0015] Table A shows that by the addition of very small quantities of a base it is possible to check the undesirable rise in temperature for a long period of time and to limit the decomposition of the hydroperoxide. In that period the cause of the rise in temperature can be removed, for example by switching over to another feed.

[0016] Table B summarizes the results of an experiment in which the oxidation of ethylbenzene was repeated in the manner described in this Example, but no NH₃ was added. No measures were taken to reduce the temperature of the reaction mixture and the passing through of the oxygen-containing gas was continued during the entire test.

[0017] Table B shows that if no NH₃ is added, the greater part of the ethylbenzene hydroperoxide decomposes.

EXAMPLE II

[0018] In the manner described in Example I ethylbenzene, to which 1.5 ppm of sulphur had been added as 2,5-dimethylthiophene, was oxidized at a temperature of 150⁰C and a pressure of 3 bar abs. After a reaction time of 90 minutes the temperature started to rise and was 152°C after a reaction time of 106 minutes. At this moment 10 ppm of NH₃ (based on the weight of the reaction mixture) were added to the mixture of air and nitrogen and the reaction mixture was cooled. The results are summarized in Table C.

[0019] Table C shows that by a single addition of a very small quantity of a base and coolingcf the reaction mixture the undesirable rise in temperature can be controlled and the decomposition of the hydroperoxide can be limited.

EXAMPLE III

[0020] In the manner described in Example I ethylbenzene, to which 1.5 ppm of sulphur had been added as 2,5-dimethylthiophene, was oxidized at a temperature of 150°C and a pressure of 3 bar abs. After a reaction time of 84 minutes the temperature began to rise and was 152⁰C after 95 minutes. At this moment 20 ppm of NH₃ were added to the gas stream and the reactor was cooled. Three minutes after the NH₃-injection the passing through of the mixture of air and nitrogen was discontinued and a weak nitrogen stream was passed through the reactor instead. The results are summarized in Table D.

[0021] Table D shows that the formation of phenol by decomposition of the hydroperoxide can be practically stopped by adding a small quantity of a base, passing no further oxygen-containing gas through the reaction mixture and cooling the reaction mixture.

[0022] Repetition of the test with 10 ppm of NH₃ yielded practically the same results.

[0023] Table E summarizes the results of an experiment in which the oxidation of ethylbenzene was repeated in the manner described in this Example, but no NH₃ was added. After a reaction time of 95 minutes the reaction mixture was cooled and after 97 minutes instead of the mixture of air and nitrogen a weak nitrogen stream was passed through the reaction mixture.

[0024] Table E shows that in spite of cooling and closure of the air stream the decomposition of the hydroperoxide continues.

[0025] Table F summarizes the results of an experiment which was carried out in the same manner and in which no NH₃ was added, the reaction mixture was not cooled and after a reaction time of 110 minutes instead of the mixture of air and nitrogen a weak nitrogen stream was passed through the reaction mixture.

[0026] Table F shows that in spite of the closure of the air stream the decomposition of the hydroperoxide continues.

EXAMPLE IV

[0027] In the manner described in Example I ethylbenzene, to which 1.5 ppm of sulphur had been added as 2,5-dimethylthiophene, was oxidized at a temperature of 150 C. After a reaction time of 80 minutes the temperature started to rise and since the reaction mixture was slightly heated in order to compensate for the loss of heat to the surroundings, the temperature reached a value of 162°C after a reaction time of 120 minutes. At this moment 10 ppm of NH₃ were injected into the mixture of air and nitrogen and the temperature immediately started to fall. The reaction mixture was cooled to 140°c in 20 minutes and maintained at 140°C for a further hour. During the cooling and the subsequent hour a constant stream of a mixture of air and nitrogen was passed through the reaction mixture. The ratio between the quantities of air and nitrogen was adjusted in such a way that the oxygen concentration in the off-gas was 4% by volume. The results are summarized in Table G.

[0028] This test shows that it is also possible to regain control of a runaway up to a relatively high temperature (162°C) by the addition of a small quantity of base.

EXAMPLE V

[0029] In the manner described in Example I ethylbenzene, to which 1.5 ppm of sulphur had been added as 2,5-dimethylthiophene, was oxidized at a temperature of 150^oC. To the mixture of air and nitrogen 0.25 nml of gaseous NH₃ was added every 2.5 minutes, so that after 2 hours 10 ppm, based on the ethylbenzene, had been added. Table H shows that a runaway cannot be prevented by this semi-continuous addition of NH₃.

Claims

1. A method for the prevention of disturbances and/or the effects of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen or a molecular oxygen-containing gas at elevated temperature, characterized in that if an uncontrolled rise in temperature occurs in performing the oxidation, a basic substance is introduced into the reaction mixture.

2. A method as claimed in claim 1, characterized in that the quantity of basic substance is between 0.05 and 20 gram-equivalents, in particular between 0.1 and 5 gram-equivalents, per 1000 kg of the reaction mixture present in the reactor.

3. A method as claimed in claim 1 or 2, characterized in that the basic substance is completely mixed with the reaction mixture within 5 minutes.

4. A method as claimed in claims 1-3, characterized in that the basic substance is added to the oxygen or the oxygen-containing gas.

5. A method as claimed in claim 4, characterized in that the basic substance is vaporous at room temperature or at least at the temperature at which the oxygen or the oxygen-containing gas is introduced into the reaction mixture.

6. A method as claimed in claim 5, characterized in that the basic substance is NH₃.

7. A method as claimed in claim 6, characterized in that the quantity of NH₃ is between 1 and 300, in particular between 2 and 80 ppmw, based on the weight of the reaction mixture present in the reactor.

8. A method as claimed in claims 1-7, characterized in that in the event of the uncontrolled rise in temperature cooling systems present in the reactor are also started up and/or the the supply of oxygen or the oxygen-containing gas is reduced or closed.

9. A process for the preparation of a hydrocarbon hydroperoxide in which oxygen or a molecular oxygen-containing gas is passed through a hydrocarbon at elevated temperature, characterized in that in order to prevent disturbances and/or the effects of disturbances a basic substance is introduced into the reaction mixture if in the course of the reaction an uncontrolled rise in temperature takes place.